Helper Cell-Independent Cloned Cytolytic T Lymphocytes. The existence of functionally distinct, interacting T cells has been well documented at both the population and clonal levels; recent results from our laboratory indicate that, at least in some cases, the progeny of a single T cell can express functions previously considered unique to different T cell subsets. More specifically, we have isolated a clone of alloreactive murine cytolytic T lymphocytes (CTY) which, unlike "conventional" CTL clones, does not require helper T cells or their factors for proliferation and manifestation of cytolytic potential in response to antigenic stimulation and which secretes a lymphokine(s) which is active in a T cell growth factor assay. We refer to cells of this phenotype as helper cell-independent CTL. A major thrust of the proposed research made possible by the ability to clone cells will focus on the isolation and extended functional categorization of additional cells of this category. The approach will be to examine cloned cells derived from several different populations whose activation requirements are not yet clear and in which there is reason to believe that helper cell-independent CTL, or other types of heretofore undefined cell types, may contribute to the response. The populations include those responsive to H-2K(b) mutant alloantigens, minor histocompatibility antigens, allogenic class II antigens, and in vivo versus in vitro primed populations. It is anticipated that investigation of the functional and cell surface characteristics of cloned T cells obtained from these sources will contribute important further information regardig the cell types involved in the immune responses. Additional work will focus on determining the potential usefulness of the Ly-5 antigenic series as a differentiation marker for functionally distinct T cell clones.